Nanotubes, in particular carbon nanotubes (CNTs), are essentially one-dimensional metallic or semiconducting materials. The electrical properties of nanotubes are dependent on, amongst others, their dimensions and orientation. When properly formed and oriented, nanotubes provide great electrical current handling capability, high thermal conductivity and high mechanical strength. These properties make nanotubes an ideal material for molecular or nano-scale electronic devices. Examples of electronic devices that have been implemented using CNTs include CNT transistors and CNT sensors.
In the past, nanotubes in the devices were formed by spin-coating or other like techniques. The formed nanotubes were randomly distributed, highly entangled and had unpredictable electrical properties. More recently, however, nanotubes have been controllably assembled into hierarchical arrays. Such assembly is typically done using methods that fall in one of two categories: in situ synthesis and post synthesis. In situ synthesis methods typically involve catalyst patterning and chemical vapour deposition (CVD). However, their high processing temperature (typically >800° C.) significantly limits the choice of substrate materials. Furthermore, the as-grown nanotubes usually have many defects and, as such, have varying conductivity.
Post-synthesis methods are typically solution-based methods. One example method that is specific to nanowires is disclosed in an article entitled ‘Directed Assembly of One-Dimensional Nanostructures into Functional Networks’ by Lieber et al (Science, 291, 630-633 (2001)). In that method, a substrate is first chemically patterned to have amine-functionalized nanopatterns and bare methyl-functionalized areas. The pattern is made using electron beam lithography and immersing the substrate in a solution of 3-Aminopropyltriethoxysilane (APTES). A micropatterned polydimethylsiloxane (PDMS) mold is then provided over the substrate such that a channel is formed between the mold and the substrate. A nanowire suspension is then controllably flowed inside the channel to assemble the nanowires into arrays of individual nanowires. The method of Lieber et al is, however, not suitable for nanotubes because it relies on the nanowires' inherent rigidity, which nanotubes do not have. The unsuitability of the method of Lieber et al for nanotubes has been noted in an article entitled ‘Self-Assembled, Deterministic Carbon Nanotube Wiring Networks’ published by Heath et al (Angewandte Chemical International Edition, 41, 351-356 (2002)). In particular, Heath et al noted that Lieber et al used rigid nanowires with controlled length and diameter, and that these were not conditions that could be replicated for nanotubes due to the flexibility and lack of dimensional control of individual nanotubes.